Geological prospecting process and apparatus



Oct. 23, 1962 3,060,371

J. TOWNSEND ETI'AL GEOLOGICAL PROSPECTING PROCESS AND APPARATUS FiledJuly 20, 1955 Ti-EH.

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FREQUENCY OF H, FIELD SIGNAL AMPLITUDE STRENGTH OF Hp FIELD NV E N TO RJuazikam Tow/ism Egg Commonefi TIME ATTORNEY Oct. 23, 1962 J. TOWNSENDETAL 3,060,371

GEOLOGICAL PROSPECTING PROCESS AND APPARATUS 4 Sheets-Sheet 2 Filed July20, 1955 MQDFIQE 44206 Ho CURRENT ATTORNEY Oct. 23, 1962 J. TOWNSEND,ETAL 3,060,371

GEOLOGICAL PROSPECTING. PROCESS AND APPARATUS Filed July 20, 1955 4Sheets-Sheet 3 RECTIFIER A 29 .31 .55 RECTIIFIER I INPUT \n r b'g'AMPUFIER l FREQUENCIES :5 Fan d T FREQUENCY T P SIGNAL fifi of m aDEMODUIATOR LOW-PASS RECTiFlER FILTER 3 o 3.

. MIXER-P m qrs- FROM 'sou'Rc-E-W IN ENTORS .fo a/ms'end 23 Com/zen Oct.23, 1962 GEOLOGICAL PROSPECTING PROCESS AND APPARATUS Filed July 20,1955 4 Sheets-Sheet 4 l CARBONACEOUS DEPOSIT CD (I II I I I INVENTORSfanaikam 76101250120 I 01719 Commoner REFLECTING Y INTERFACE I mATTORNEY J. TOWNSEND ETAL 7 3,060,371

United atent 3,060,371 GEOLOGICAL PROSPECTING PROCESS AND APPARATUSJonathan Townsend, 5942 Horton Place, St. Louis 12,

Mo., and Barry Commoner, 50 Arundel Place, Clayton, Mo.

Filed July 20, 1955, Ser. No. 523,213 26 Claims. (Cl. 324.5)

This invention relates to the detection of carbonaceous materials andmaterials geologically associated therewith. More particularly itrelates to the location of subterranean or inaccessible geologicaldeposits of carbonaceous materials and materials often associatedtherewith, such as uranium. By carbonaceous materials as used herein ismeant petroleum, coal, natural bitumens including tars and asphalts,partially carbonized animal and vegetable matter and carbonaceousgeological deposits and formations including oil-bearing shales.

As is well known, valuable deposits of carbonaceous materials may occurin subterranean locations which become accessible only through drillingor mining operations. Uranium ores also may occur in subterraneandeposits. Such uranium deposits are frequently found in association withcarbonaceous materials including partial ly carbonized animal andvegetable matter, bituminous materials, such as pitchblende andgilsonite, and carbonaceous shales. Typical of this relationship is thepresence of uranium salts among fossilized plant materials or otherorganic geological debris collected in depressed areas, such as inancient stream beds, swamps or shoreline areas. For example, this typeof geological situation is characteristic of the uranium-bearingMorrison, Entrada and Shinarump formations of Utah and Colorado.

Prior to this invention there has been no way of directly and positivelydetecting from a distance the presence of subterranean deposits ofcarbonaceous materials, such as petroleum, coal, and the othercarbonaceous materials hereinabove identified. In the case of petroleum,for example, apart from the expensive method of boring or digging intothe earths crust, most, if not all, other methods used for locatingdeposits of petroleum indicate only the presence of geologicalstructures capable of trapping petroleum, but leave open the questionwhether the traps actually contain the material. Such other methodsinclude surface observations of geological structures and electrical,seismic, gravity, magnetic, and radioactive methods. The electricalmethods usually depend upon differences of electrical conductivity ordielectric properties between the deposit sought and the surroundingearth, use electrical signals of one or more frequencies, and depend fordetection upon these same frequencies. The magnetic methods usuallydepend upon a distortion of the earths magnetic field by iron-containingcompounds in parts of the geological structures.

Prior to this invention, the art of detecting useful undergrounddeposits of uranium has also been based chiefly on indirect geophysicalmethods which describe the distribution of geological formationsbelieved to contain uranium. The direct method of detecting uranium oresby means of the gamma rays which they emit is effective only inconnection with uranium ore deposits located at or very near the surfaceof the earth. How ever, most of the useful ore deposits and particularlythose in the United States of America are located in scattered depositsat depths of fifty to two hundred feet more or less. Gamma radiationwill not penetrate through such a thickness of overlying soil andtherefore cannot be used for direct localization of these subterraneanuranium deposits. Gamma radiation detectors serve only to locateoutcroppings of uranium ore which may bear some stratigraphicrelationship to other deposits lying at a depth. Thus, even where avaluable outcropping has been located by gamma radiation detection, theproblem still remains to find other nearby deposits which lieunderground. Prior to this invention this has been accomplished bydrilling numerous and expensive bore holes in the neighborhood of thedetected outcropping in an endeavor to find useful ores at a depth.Since even near an ore outcropping the underground deposits may bescattered at random, the present methods require a great deal ofdrilling, much of which is fruitless.

It is among the objects of this invention to provide process andapparatus for detecting directly the presence of inaccessible geologicaldeposits of carbonaceous materials and/ or of uranium found inassociation therewith.

A further object is to provide such process and apparatus for detectingdirectly the presence of carbonaceous materials and/ or uranium found inassociation therewith in a mixture of such materials with othermaterials and where the presence of the carbonaceous materials is notreadily apparent.

Still a further object is to provide such process and apparatus fordetecting, locating and determining the size of subterranean deposits ofcarbonaceous materials to the end that geographical distribution of thedeposits may be mapped and drilling operations guided by the informationthus obtained.

Still a further object is to provide process and apparatus for locatingscattered underground uranium deposits found in association withcarbonaceous materials, eliminating the drilling of unnecessary orprofitless bore holes in the location of such uranium deposits.

Still another object of the invention is to provide electricalgeophysical exploration method and apparatus for locating deposits ofcarbonaceous materials and/or of uranium associated therewith whichmethod and apparatus is more certain in its performance than prior knownmethods and apparatus.

Other objects and advantages of this invention will be apparent from thefollowing detailed description thereof.

In accordance with this invention, inaccessible deposits of carbonaceousmaterials and/or of uranium associated therewith are located byutilizing or creating, in the subterranean zone or region being exploreda magnetic field, desirably a unidirectionally pulsating magnetic field(such field being herein for convenience referred to as the H field),creating an alternating or rotating magnetic field in such subterraneanzone or region (such field being herein for convenience referred to asthe H field), which field is transverse and usually at right angles tothe H field, with a frequency (1) related to the strength of the H fieldin accordance with the formula ;f=KH where K is about 2.8 megacycles persecond per gauss, or in accordance with the formula In the above formulaand throughout the specification the frequencies referred to are inmegacycles per second; the strengths of the magnetic fields are given ingauss. The invention also involves noting, detecting, or measuring at aremote point magnetic changes resulting from the presence ofcarbonaceous material in the subterranean zone or region being explored,if such material is present, as by utilizing a detector actuated byenergy from the H field to give a signal or otherwise indicate thepresence of such carbonaceous material. The above Formula 1 representsthe limits of the broad range of frequencies of the H field andstrengths of the H field; the limits of the preferred range areindicated by the formula The H, field may be the earths magnetic field,a component thereof, or a specially created magnetic induction field ormagnetic component of a radiation field as hereinafter more fullydescribed. Induction fields do not permanently leave the regionsurrounding the apparatus as do those fields classed as radiationfields. The latter become detached from the apparatus and propagatethemselves away indefinitely until reflected, refracted, or absorbed byobstacles. Radiation fields consist of an inseparable mixture ofelectric and magnetic fields, both of which are usually perpendicular toeach other and to the direction of propagation. A beam ofelectromagnetic radiation in which the electric fields are oriented inone direction is called a polarized beam, and the direction of theelectric fields is called the direction of polarization.

The H field may also be of the induction or radiation type and shouldhave a frequency of from 0.1 to 10,000, preferably from 0.25 to 250megacycles per second, the latter range of frequencies beingparticularly useful when employing this invention for geologicalprospecting.

An alternating magnetic field of the induction type may be produced by acoil supplied with an alternating current. A rotating magnetic field maybe produced by superimposing two alternating magnetic fields, mutuallyperpendicular, and having a constant phase difierence of 90 orone-fourth of a cycle.

In accordance with a preferred embodiment of this invention either thestrength of the H field or the frequency of the H field is modulated inthe region of resonance to improve the clarity of the signal or othersign of detection given by the detector. Best results are obtained bymodulating the strength of the H field in the region of resonance. Bythe region of resonance is meant the region of the above mentionedformulas represented by the range of plus or minus 10, preferably plusor minus 2.5 gauss. Stated otherwise, the region of resonance, in thecase of modulation of H field strength, is the range of values of Hfield strength in which the ordinates of the curves of FIGURES 1 and 2,hereinafter more fully described, are appreciably different from zero.In the case of modulation of the H field frequency the region ofresonance is the range of values of the H field frequency of the curvesof FIGURES 3 and 4, hereinafter more fully described, in which theordinates of these curves are appreciably different from zero.

With the thought that it would be helpful to a better understanding ofthe invention, the following explanation and description of certainscientific principles used in the invention are believed to beexplanatory of its unexpected results, and are given before proceedingto a detailed description of the drawings and of exemplary embodimentsof the invention. It will be understood, however, that this explanationand discussion, as noted, is advanced in the interests of facilitating abetter understanding of the invention and the invention is not to belimited by this explanation.

The discussion which follows will use the terms of classical physicsrather than those of quantum physics because it is recognized that inthe subjects to be discussed, both lead to the same result and classicalphysics is more familiar to the general reader.

In the course of research conducted by us it was discovered thatcarbonaceous materials hereinabove enumerated are exceptional ascompared with other materials in that they have a molecular structurecomprising a small fraction of unpaired electrons. Concentrations ofunpaired electrons in coal and petroleum samples tested by us ranged upto 0.001 mol per liter.

As is well known, electrons are magnetic. If an isolated electron beplaced in a magnetic field, it will experience a torque tending to alignits magnetic axis parallel to the direction of the magnetic field, inthe manner of a compass needle in a magnetic field.

In addition, each electron behaves as if it were spinning about itsmagnetic axis. A spinning object, whose axis is acted on by a torque,undergoes precession, the familiar motion of a spinning top orgyroscope. Hence, an electron placed in a magnetic field does notimmediately align its axis parallel to the field "but undergoes aprecession in which its axis maintains a nearly constant angle betweenitself and the direction of the magnetic field. The frequency of thisprecession depends upon the magnetic moment, the angular momentum of theelectron and the strength of the magnetic field. Since all electrons areknown to have identical properties, all electrons precess at the samefrequency in the same magnetic field. This frequency, often called theLarmor frequency for electrons, is given by the equation where is inmegacycles per second, and H is the strength of the magnetic field ingauss. Stated otherwise, the rate, or frequency, of precessionisproportional to the strength of the magnetic field, being about 2.80megacycles per second per gauss of field strength.

Most matter is so constituted that the electrons occur in pairs in sucha way that the magnetic properties of each electron are cancelled bythose of its partner. The electrons of most substances therefore do notpartake of the action described above. As above noted, the carbonaceousmaterials, hereinabove set forth, are unusual or exceptional in thatthey have a small fraction of their electrons not paired.

When a carbonaceous material is placed in a magnetic field its unpairedelectrons are acted on by the field, as described above, and also areacted on by dissipative forces, which transfer the energy of precessioninto heat energy. As this happens, the axes of the electrons become morenearly parallel to the direction of the magnetic field, until finallycomplete alignment occurs and precession ceases to exist. It ispossible, however, to supply, from an outside source, energy to start orsteadily maintain the precession despite the effects of dissipativemechanisms. This is done by the second magnetic field, H orientedperpendicularly to the first, and either rotating or oscillating at afrequency f, equal to or near the Larmor frequency f The first magneticfield, whose direction is fixed, is the H field, above mentioned, andthe second rotating or oscillating field is the H field, abovementioned. The agency supplying the H field is required to furnishenergy to the processing electrons, which, in turn, pass the energy,through the dissipative mechanisms mentioned, into heat energy. Thisphenomenon is termed resonance absorption. If the agency producing the Hfield is a current-carrying coil, then the phenomenon of resonanceabsorption increases the loss factor of the coil.

In the phenomenon of resonance, the unpaired electrons are precessingpredominantly in unison, that is, their axes predominantly point in thesame direction at any given instant. Their individual magnetic momentsthen add to produce an overall magnetic moment of the substance. Thisoverall magnetic quantity precesses about the direction of the H field,and does so at the same frequency as the applied H field. The absorptionof energy from the H field may be viewed as being a result of the factthat the precessing magnetic moment vector adjusts itself so that it hasa component opposite in direction to the vector representing the timerate of change of the H field.

By measuring the power absorbed from the H field, while holding itsamplitude and frequency f constant but varying the Larmor frequency f byvarying the H field strength, in accordance with Equation 3, theresonance curve of FIGURE 1 of the later described drawings is obtainedwhen a carbonaceous substance is in the fields.

The value of H at the center of the curve is that given by Equation 3together with the resonance requirement f =f. The width, AH of theresonance curve,

depends upon the strength of the dissipative mechanisms mentioned above.We have found that for representative samples of coal and crude oil thevalue of AH, is is approximately gauss.

The precessing magnetic moment vector also has, 1n general, a component,either positive or negative, in the direction of the H field vector. Theeffect of this is to change the magnetic susceptibility of the samplewith respect to the H field. The behavior of this susceptibility as afunction of the strength of the H field is depicted in FIGURE 2. Thischange of susceptibility will hereinafter be called resonancedispersion. If the agency producing the H field is a current-carryingcoil, then the phenomenon of resonance dispersion effects the inductanceof the coil.

Still another phenomenon may be observed by creating an oscillating Hfield in a direction perpendicular to the H field and establishing adetector which will detect a component of the magnetic momentoscillating in a direction perpendicular to both the H and H fields. Aplot of the amplitude of oscillation of this component of the magneticmoment as a function of the strength of the H field will closelyresemble the curves of FIG- URES l and 3. This phenomenon will behereinafter called resonance induction.

A beam of radiation, having a frequency f, travelling through acarbonaceous material can be affected by resonance in the material if asteady field H the strength of which lies in the region of resonance,exists. For example, if the H field is in the same direction as thedirection of propagation of the radiation, a progressive rotation, knownas Faraday rotation, of the direction of polarization occurs. If the Hfield is in the direction of the electric field of radiation, then achange of wave length of the radiation occurs if the H field passesthrough the resonance range. In both of these cases there is also aprogressive attenuation of the beam due t resonance absorption. If the Hfield is in the direction of the magnetic field of radiation, then nointeraction occurs.

One embodiment of our invention for the detection of an inaccessibledeposit of carbonaceous material involves: (1) a source of alternatingelectrical power communicating with a system of coils or antennaspowered by the source which establishes an alternating (or rotating)magnetic field at a frequency f, the H field, at right angles to the Hfield which may be the earth's magnetic field or a magnetic fieldspecially created, both fields being in the locality of exploration; and(2) a suitable detector actuated by energy from the H field when acarbonaceous substance is present in both fields.

In a typical case, a plot of the amplitude of the signal given to theabove mentioned detector vs. the H field strength may appear as curve 10in FIGURE 5.

Such a curve may be plotted by taking successive readings of thedetector at progressively higher values of H However, the sensitivity ofthis method is limited by disturbances and fluctuations of the apparatuswhich occur between the times at which the readings are taken.

An enhancement of detection sensitivity may be obtained by superimposingupon the constant or slowly varying H field strength a sinusoidalvariation with time, as shown in curve 12, FIGURE 5, or by modulatingthe frequency of the H field in the region of resonance. Such avariation, or modulation, of the H field strength or of the H fieldfrequency will produce an amplitude modulation of the signal to thedetector, as shown in curve 13, FIGURE 5. The modulation envelope,represented by curve 13, may consist predominantly of a frequencycomponent of the same frequency as the modulation of H as shown, or, ifthe modulation of H, has a greater amplitude, causing H to oscillateover most of the resonance region, higher harmonics may predominate inthe modulation envelope.

It is a fact well known in the art of electrical communication that asignal of frequency f which has undergone amplitude modulation no longerconsists of a single frequency, but has other frequency components, orsidebands, two for each frequency component in the modulating signal.The sidebands of a component of the modulating signal of frequency 111have frequencies f-l-m and m, respectively.

When the H field (but not the frequency of the H field) is modulated ata frequency In, the detector receives a modulated wave having, ingeneral, frequencies f, f+m, f-m, f+2m, f2m, etc., in the presence ofresonance and only the frequency f in its absence. A suitable detectorcould then have two possible forms, with respect to frequencyconsiderations: (1) a demodulator acting upon the total input signal,producing a signal at the modulation frequency plus its harmonics,followed by a filter selecting one of these frequencies to provideindication; and (2) a filter selecting one of the sideband frequenciesto produce indication directly. In either case, the presence of sidebandfrequencies is necessary for the detector to indicate resonance. Thesesideband frequencies are produced not by the apparatus but by thecarbonaceous material itself.

In the accompanying drawings, forming a part of this specification andshowing, for purposes of exemplification, preferred forms of thisinvention Without limiting the claimed invention to such illustrativeinstances:

FIGURE 1, as noted, is a typical resonance curve in which the powerabsorbed from the H field is plotted against the strength of the H feld;

FIGURE 2 is a curve showing the change in the magnetic susceptibility tothe H field as the strength of the H field is varied;

FIGURE 3 is a typical resonance curve in which the power absorbed fromthe H field is plotted against the frequency of the H field;

FIGURE 4 is a curve showing the change in the magnetic susceptibility tothe H field as the frequency of the H field is varied;

FIGURE 5 shows plots of amplitude of the signals with variation in thestrength of the H field;

FIGURE 6 illustrates apparatus embodying this invention, forprospecting, by means of resonance absorption or dispersion, usingelectromagnetic induction fields;

FIGURE 7 is a block diagram showing the relationship between the wellknown parts of one form of detector;

FIGURE 8 illustrates a modified form of apparatus embodying thisinvention, for prospecting by means of resonance induction, usingelectromagnetic induction fields;

FIGURE 9 illustrates still another modified form of apparatus forprospecting by resonance absorption or dispersion, using anelectromagnetic radiation field for H and an electromagnetic inductionfield for H FIGURE 10 illustrates still another modified form ofapparatus for prospecting, in accordance with this invention, andinvolves the use of Faraday rotation, an electromagnetic radiation fieldfor H and an electromagnetic induction field for H FIGURE 11 illustratesstill another modified form of apparatus for prospecting, in accordancewith this invention, by means of resonance induction usingelectromagnetic radiation fields for both H and H FIGURE 12 illustratesan embodiment of this invention for logging a bore hole by means ofresonance induction, using electromagnetic induction fields; and

FIGURE 13 shows plots of amplitude of the signals with variations in thevalue of current which flows in the coil which generates the H field.

In the several figures of the drawings, like parts are indicated by thesame reference characters. Referring to FIGURE 6, a coil 15 is placed atthe surface of the ground with axis vertical and is energized by asource of direct current 16 and a source of alternating current 17. Thiscoil produces a magnetic field, the H field, beneath the surface,represented by arrow H on FIGURE 6 in the vertical direction. Thecurrent supplied by source 16 is varied until the magnetic field at agiven selected depth of exploration is brought into the region ofresonance of petroleum or other carbonaceous substance at the selectedfrequency f, of operation (given by Equation 3 above). The alternatingcurrent source 17 is adjusted to provide a suitable modulation of H asexplained above in connection with FIGURE 5. Let the modulationfrequency be called m.

The two coils 19, 20, which are symmetrically located with respect tothe center of coil and connected in such a way as to produce magneticfields at their centers having opposite directions, are employed toproduce the H magnetic field below the surface 21 in a horizontaldirection. The directions of the H and H fields are indicated by thearrows on FIGURE 6.

Coils 19, are energized, at the frequency f, by source 22 throughbridge-arm impedances 23, 24 and 25. The condenser 26, together withcoils 19, 20, form a circuit resonant at the frequency 1, but theresonance is broad enough to transmit the sidebands at frequencies fmand f-l-m. Impedances 23, 24 and 25 may be adjusted, by well knownmethods, so that the signal fed to detector 27 is determined by (a)changes of the loss factor of coils 19, 20 or (b) changes of theinductance of coils 19, 20. As previously explained, (a) then permitsdetection of resonance absorption, and (b) permits detection ofresonance dispersion. The detection sensitivity achievable will not beappreciably different for the two cases.

In FIGURE 7, is shown one form of detector designed to detect sidebandsin the signal having frequencies f-m and f-l-m. As shown in FIGURE 7, apreferred arrangement for this detector consists of an amplifier 28 toamplify the frequency f and the sidebands, a demodulator 29 to produce asignal of frequency m from the sidebands, a further amplifier 31 toamplify the frequency m, a suitable mixer 32 to mix this signal togetherwith one derived directly from source 17 circuit to produce azero-frequency signal, and a low-pass-filter 33 to remove as much noiseas possible by limiting the effective noise band-width of the system toa suitable low value. The resulting output is a D.C. signal the value ofwhich is indicative of the presence of resonance, e.g., resulting from adeposit of carbonaceous material at the selected depth. As these partsare well known in the electronic art, further description thereof isbelieved unnecessary. This arrangement of the detector, and also thearrangement of the bridge circuit consisting of impedances 23,

24 and 25 are illustrative, and other well known means of accomplishingthese functions could be employed. For convenience the reference numeral27 is used to indicate a detector in the several views. It Will be understood, however, that any suitable form of detector may be used anddifferent forms of detectors may be used in the different modifications.

A variation of the arrangement of FIGURE 6 may be had by omittingdirect-current source 16 and depending upon the earths field for theconstant part of H The frequency i must then be chosen accordingly. Thisvariation results in a saving of the power otherwise needed for source16. A further saving of power may be made by replacing the sinusoidalsource 17 by 'a pulse generator furnishing short pulses of largecurrent. In this case the frequencies supplied by source 17 consist ofthe pulse repetition frequency m, plus many harmonics or multiples of m.Each harmonic, including the frequency In as well as its multiples,produces a pair of sidebands in the received signal. The detector may bemade to re spond to one or more of these sidebands.

The roles of the coil 15 and the pair of coils 19, 20 can beinterchanged, thereby producing a horizontal H field and a vertical Hfield without substantially altering the operation or result.

Alternatively, source 17 may be omitted, thus employing a: constant Hfield produced by direct-current source 16, and the frequency of the Hfield modulated through the region of resonance by proper variation ofthe frequency of the current produced by source 22. A current of varyingfrequency in the ranges given by the above Formulas l or 2, i.e., in theregion of resonance, may be produced by any means well known in theelectrical field and accordingly a description of such means would serveno useful purpose.

Referring now to FIGURE 8, which is a diagram of apparatus forexploration by resonance induction, coil 15 and power sources 16 and 17perform the same functions as in FIGURE 6, namely, the production of amodulated H field. The coils 19, 20 and source of alternating current 22also serve, as in FIGURE 6, to produce an H field, indicated by thearrow on FIGURE 8, oscillating at the desired operating frequency, f. Ifthere is present a carbonaceous substance, a precessing magnetic momentis produced, which will have an oscillating component perpendicular to Hand H and indicated by the arrow 35 on FIGURE 8. This component willinduce a voltage in coils 36, 37, which are similar to coils 19, 20except that they are oriented in such manner that a line joining theirrespective centers is at right angles to a line joining the respectivecenters of coils 19, 20, in order to minimize the direct induction of aVoltage in coils 36 and '37, due to the current in coils 19, 20.Detector 27 is connected to coils 36, 37; the description of thisdetector is given above in connection with the description of FIGURES 6and 7.

The functions of coils 15, 19, 20, 36 and 37 could be permuted in anymanner with substantially equal results. As in the case of FIGURE 6hereinabove described, source 17 may be omitted and the H fieldfrequency modulated (instead of the strength of the H field effected bysource 17) by proper variation of the current generated by source 22.

In FIGURE 9 is shown apparatus employing an induction field for H and aradiation field for H Coil l5, D.C. source 16 and A.C. source 17 producea vertical modulated H field, as in FIGURE 6. The H field is produced bya directional antenna 49, which, for convenience of illustration, isshown as a dipole antenna 41 with a parabolic reflector 42, although anyof the types of directional antennas known to the art of radiocommunication might be used. The antenna directs a beam of radiationinto the earth at the center of coil '15.

Antenna 40 is energized by A.C. source 22 through a bridge circuitconsisting of impedances 23, 24 and 25. Detector 27, of the typediscussed in connection with FIGURE 7, is connected to the bridge asshown.

The beam of radiation penetrates the earth and is partially reflected atinterfaces between layers of earth having different electricalproperties. If it passes through a carbonaceous material of the typehereinabove identified, it will suffer an absorption, the value of whichwill depend upon the value of the H field, and the radiation reflectedfrom lower interfaces will suffer further absorption on its way back tothe antenna. This radiation, now modulated by resonance absorption,enters antenna 40 and actuates detector 27. The bridge circuit isbalanced so that the power entering the detector directly from source 22is minimized.

FIGURE 10 shows an arrangement for making use of the Faraday rotation ofthe plane of polarization of a beam of radiation by carbonaceousmaterials in accordance with our invention. As before, coil 15 andsources 16 and 17 create a modulated H tfield in the vertical direction.Source 22 energizes a dipole antenna 46, which, together with parabolicreflector 44, emits a beam of radiation downward into the earth. In thepresence of a carbonaceous material the direction of polarization isrotated, the direction of rotation depending upon the direction of the Hfield. A part is reflected upward from some lowerlying interface, againpasses through the material, and is rotated further in the samedirection. The rotated beam enters the antenna and induces a signal intothe dipole antenna 45, which is perpendicular to antenna 46, andtherefore has little direct coupling to it. The signal induced inantenna 45, which is modulated at the frequency of source 17, isdetected by detector 27, which is similar to that of FIGURE 7. Insteadof the set of dipole antennas with a parabolic reflector, other wellknown directional antennas may be used.

FIGURE 11 shows an arrangement for prospecting by means of radiationfields for both H and H The H field consists of a steady component,contributed by the earths magnetic field, plus an alternating component,furnished as a radiation field by directional antenna 50 and generator51. Preferably, antenna Stl is so oriented that the magnetic componentsof its radiation field are substantially parallel to the earths field.

The H field is the magnetic component of the radiation field created bydirectional antenna 52, which is powered by generator 53. Antenna 52should be oriented so that the field is substantially perpendicular tothe H field.

The beam of radiation from antenna 52. is reflected to the surface byone or more interfaces between subterranean layers of differentelectrical properties. A part of it is intercepted by antenna 54 and thesignal is fed to detector 27, which is similar to the detector shown inFIGURE 7. Antenna 54 and detector 27 are designed to detect radiationwith frequency lying in one of the sidebands, preferably at f+m or f-m,where f is the frequency of source 53, and m is the frequency of source51. These sidebands are the result of amplitude modulation produced by adeposit of a carbonaceous material in the earth. The amplitudemodulation may arise in one or both of two ways, depending upon thegeometrical relationships.

One way is due to absorption of the electromagnetic wave as it passesthrough the deposit. The amount of this absorption is varied by themodulation of the H field. The second Way is due to a variation in therefractive index of the deposit of carbonaceous material, also producedby the modulation of the H field. The beam of radiation will bedeflected through certain angles upon centering and leaving the deposit,and these angles will vary with H Hence, the position and possibly thedirection of travel of the emergent beam will vary with H If antenna 54is placed near the edge of the reflected beam, any small variation ofthe position of the beam will produce an amplitude modulation of thesignal to detector 27.

FIGURE 12 illustrates a device to be lowered into a bore hole forlocating deposits of carbonaceous materials, particularly petroleum,near the hole at various depths. The principle of operation of thisdevice is identical to that of the device of FIGURE 8, and correspondingparts bear the same numbers. The coils are smaller, and are arranged,with axes mutually perpendicular, upon a cylindrical support 60 ofsuitable size to be lowered into a hole. The connections are madethrough cables 61.

The device of FIGURE 12 is particularly useful in locating deposits ofpetroleum and of other carbonaceous materials laterally near the borehole but through which the bore hole passes. It is not uncommon in thedigging of wells in an endeavor to locate underground deposits ofpetroleum, for the bore hole to pass through a deposit of petroleumlocated laterally adjacent the bore hole but separated from the borehole by rock or other earth formations which prevent the petroleum fromentering the bore hole. The device of FIGURE 12 gives a direct positiveindication of the presence of such deposits within an appreciabledistance laterally of the bore hole. Also the device may be lowered tothe bore of a dry well to give an indication of the presence ofcarbonaceous material including petroleum below the base of the well andthus indicate whether or not it is desirable to deepen the bore hole.

In determining the east-west and north-south location of a deposit ofcarbonaceous material, such as a deposit of petroleum, the set of coilsor coil and antenna of FIG- URES [6, 8, 9 and 10 are moved over thesurface, and the successive indications obtained at different positionsare noted. This will give for all practical purposes the importantconfines or border areas of the deposit. By varying the current suppliedto the coil i15, one can explore various depths of the earths surface.The regions of the earth being explored are those in which coil 15 setsup an H field in the region of resonance as previously defined; thisdepth will vary as the current supplied to the coil 15 is varied in awell known way. The arrangement of FIG- URE 11 is particularly suitedfor exploring different levels of the earths surface. This arrangement,as above noted, uses the earths field as the H field, thereby savingmuch power and also using a lower value of 1, which penetrates the earthmore readily. The antennas 50 and 52 can be made large enough to sendinto the earth narrow beams of radiation 'whose axes are shown by thedotted lines on FIGURE 11. The region of the earth being explored liesat the intersection of these beams and its location can easily be foundfrom the locations and orientations of antennas 50 and 52. By changingthe positions of the antennas 50 and 52 to cause the beams of radiationemanating therefrom to intersect at different points, the region ofexploration can readily be changed.

In the foregoing discussion, it has been assumed that the H field issubstantially uniform over the entire spatial extent of the deposit ofcarbonaceous material to be detected. By this it is meant that themaximum variation of H over the extent of the deposit is small comparedto AHO- This is true, according to electromagnetic theory, in cases inwhich the greatest linear dimension of the deposit is small comparedwith the smallest distance between a point in the deposit and a point onthe coil (or any of a set of coils) producing the H field.

Thus, the above discussion applies directly to the detection ofrelatively small deposits of carbonaceous material at relatively largedistances from the apparatus. For deposits in which this is not the caseand hence the H field is not substantially uniform over the deposit, oneof two alternative procedures may be employed, namely, (a) restrict theextent of the H field to a limited portion of the entire deposit, suchthat the H field is substantially uniform in the limited portion, or (b)not so restrict the H field but induce electron procession over a rangeof Larmor frequencies corresponding to the range of H in the deposit.These alternatives are discussed below in turn.

Alternative (a) can be practiced with the apparatus of FIGURE 6 orFIGURE 8 by making coil 15 much larger than coils r19, 20, 36 and 37.Thus the H field, which will be restricted to a region near coils 19 and20, will lie entirely within a region near the center of coil 15, inwhich the H field produced by coil 15 will be substantially uniform.

Alternative (a) can be practiced with the use of the equipment shown inFIGURE 9 or FIGURE 10 in the relatively common case in which thedeposits of carbonaceous material lie in thin horizontal veins. Here theantenna (40 in FIGURE 9, 44 in FIGURE 10) is designed to direct a narrowbeam of radiation downward. The intersection of this narrow beam ofradiation with the thin vein of carbonaceous material will outline asufficiently small region such that the H field can be kept uniformwithin its boundaries.

In FIGURE 11, the H field is the earths field which is sufficientlyuniform, with a superimposed modulation consisting of the magneticcomponent of the radiation field produced by antenna 50 and source 51.When practicing alternative (a) with the apparatus of FIGURE 11, thebeam from antenna 52 should be narrower than that from antenna 50, andthe thickness of the carbonaceous 1 l deposit should be less .than aboutone-fourth of a wavelength of the radiation from antenna 50.

Alternative (b) may be practiced using any of the apparatus of FIGURES 6to 12, inclusive; the bore hole instrument of FIGURE 10 will usually beoperated to practice alternative (b).

The following explanation should aid in understanding alternative (b).

Assume the carbonaceous deposit to be detected is divided by imaginarysurfaces into a large number of small parts, called deposit elements,such that the H field, at any instant, is substantially uniform in thesense previously described, throughout each such deposit element. (Forexample, these surfaces could be three orthogonal sets of planes whichdivide the deposit into small cubes.) The H, field at any given instantmay, of course, have appreciably different values at two widelyseparated deposit elements.

To a very good approximation, each deposit element makes a contributionto the detected signal, called a signal element, which is independent ofthe presence of the other deposit elements. In other words, the entiresignal, due to the entire deposit, is a sum of the signal elements, eachdue to a deposit element.

Referring now to FIGURE 13, in this plot the values of current whichflows in the coil which generates the H field are plotted as theabscissae (this current is called herein the H current, and it flows incoil of FIGURES 6, 8, 9, l0 and 12). In this FIGURE 13, the horizontalcurve a is simply a baseline, as in the case of the horizontal curve inFIGURE 5. A signal element due to a particular deposit element isrepresented by dip b. A second signal element, due to a second depositelement farther away from the coil which generates the H field is shownas clip 0. That this dip lies to the right of the first is, of course, aconsequence of the fact that a larger H current is required to bring theH field into the region of resonance at the greater distance of thesecond deposit element.

The fact that dip b is larger than dip c is meant to illustrate that thefirst deposit element will usually be closer to the coils which producethe H field and which detect the magnetic moment of the depositelements, and will therefore usually produce a greater signal element.

If all of the signal elements are added together on this graph, theresult is a curve such as d, representing the signal due to the entiredeposit. The exact shape of curve d depends upon a large number ofgeometrical relationships between the deposit of carbonaceous materialand the various coils. The particular curve of FIGURE 13 was drawn Withthe maximum depression nearer the lower values of the H current toillustrate the usual situation, wherein the nearer deposit elementscontribute a stronger signal than those farther awa due to abovementioned proximity effects, despite the fact that usually there arefewer deposit elements nearer the coils than farther away. In general,however, curve d may be expected to have portions in which its slope(rate of change of signal amplitude with respect to a change of Hcurrent) is not zero.

Such a portion having non-zero slope permits the use of the detectionmethodp reviously discussed, namely, a method in which the H current iscyclically varied and the corresponding cyclical variations in signalstrength are detected, as shown in FIGURE 13.

It should be clear that the invention is applicable to the directdetection of subterranean masses of carbonaceous material, whether thepurpose of the exploration is the location of such masses per se or ofuranium frequently found associated therewith. As noted, underground orotherwise inaccessible deposits of asphaltic materials like gilsonite,carbonized fossil wood, organic debris, pitchblende, and carbonizedshales, with which uranium is frequently associated, may be located bythe present invention and hence this invention can be used to detectdirectly such uranium deposits.

Moreover, while the invention has been described hereinabove and hasbeen exemplified in the drawings in connection with the location ofinaccessible deposits of carbonaceous materials and/ or of uranium foundassociated therewith, it will be understood it is not limited thereto.In the location of inaccessible deposits of carbonaceous material, theinvention involves the principle of actuating a detector employing aproperty of the carbonaceous material being sought which property ispeculiar to the material and is not possessed by other materials in theearths crust or located in the neighborhood of the carbonaceousmaterials. Hence this invention can be employed to detect the presenceof carbonaceous materials including petroleum present in a mixture withother materials, such as sand, mud, etc., where the presence of thecarbonaceous material, either because of the small amount present or forother reasons is not readily evident to the observer.

Since many apparent widely ditferent embodiments of this invention couldbe made without departing from its scope, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A process of geophysical exploration for subterranean deposits ofcarbonaceous materials, which process includes varying the relationbetween frequency and field strength of two magnetic fields intersectingin a subterranean zone of exploration, said fields being respectivelyunidirectional and alternating magnetic fields, said variation beingthrough a relationship where f is the frequency of the alternating fieldin megacycles per second, H is the strength of the unidirectional fieldin gauss, and K is approximately 2.8 megacycles per second per gauss anddetecting at a position remote from said subterranean zone magneticfluctuations resulting from the presence of such carbonaceous materialin said subterranean zone of exploration.

2. A process as defined in claim 1 including the step of varying thefield strength of said unidirectional magnetic field through saidrelationship and measuring changes in energy produced by such change infield strength as an indication of the presence of such carbonaceousmaterial in said zone.

3. A process as defined in claim 1 including the step of modulating theamplitude of said unidirectional field at a frequency m, any substantialamount of said carbonaceous material in said subterranean zone resultingin a signal with sideband frequencies f+m and fm, and detecting at aremote point the presence of said sidebands in said signal.

4. A process of geophysical exploration for subterranean deposits ofcarbonaceous materials including petroleum, coal, natural bitumensincluding tars and asphalts, partially carbonized animal and vegetablematter, and oil-bearing shales, which process comprises establishing amagnetic field of known frequency in a subterranean zone while there ispresent therein a unidirectional magnetic field, said fields crossingeach other in said subterranean zone, varying the relation betweenfrequency and strength of said fields through a range including theregion of resonance of electrons present in the carbonaceous materialbeing sought, to produce a signal which develops a resonance peak ifcarbonaceous material is present in said zone and as a result of suchpresence of carbonaceous material, and detecting the presence of saidpeak in said signal as indicative of such presence of carbonaceousmaterial in said zone.

5. A process of geophysical exploration for subterranean deposits ofcarbonaceous materials, which process comprises establishing a secondelectromagentic field at right angles to the direction of a firstmagnetic field in the subterranean region of exploration, the saidsecond field having a frequency related to the strength of the saidfirst field indicated by the formula in which formula, 1 is thefrequency of the said second field in megacycles per second and H is thestrength of the said first field in gauss, and detecting magneticfluctuations due to the presence of said carbonaceous material in thelocality being explored.

6. A process of geophysical exploration for inaccessible subterraneandeposits of carbonaceous materials, which process comprises creating inthe subterranean region of exploration an electromagnetic field having afrequency of from 0.1 to 10,000 megacycles per second at right angles toanother magnetic field having a strength indicated by the formula f=2.80[H t in which formula 1 is the frequency of the first-mentioned field inmegacycles per second and H is the strength of the second-mentionedfield in gauss, and detecting a change in the energy content of thefirst-mentioned field caused by the presence of a carbonaceous materialin said magnetic fields.

7. A process of locating an inaccessible subterranean deposit ofpetroleum, which process comprises creating in the subterranean regionof exploration an electromag netic field having a frequency of from 0.25to 250 megacycles per second at right angles to another magnetic field,the strength of which is related to the first-mentioned field inaccordance with the formula f=2.80[H :2.5] in which formula 1 is thefrequency of the first-mentioned field in megacycles per second and H isthe strength of the second-mentioned field in gauss, and detectingmagnetic fluctuations due to presence of said petroleum in said magneticfields.

8. A process of locating an inaccessible subterranean deposit ofcarbonaceous material, which process comprises creating in thesubterranean region of exploration an electromagnetic field having afrequency of from 0.25 to 250 megacycles per second at right angles to asecond magnetic field, the strength of which is related to thefirstmentioned field in accordance with the formula f=2.80[H L2.5] inwhich formula 1 is the frequency of the first-mentioned field inmegacycles per second and H is the strength of the second-mentionedfield in gauss, and detecting magnetic fluctuations resulting from thepresence of such carbonaceous material in said region of exploration.

9. A process of geophysical exploration for inaccessible subterraneandeposits of a material from the group consisting of petroleum, coal,natural bitumen, partially carbonized animal and vegetable matter andcarbonaceous geological deposits, which process comprises establishing afirst magnetic field in a subterranean locality to be explored, varyingthe strength of said magnetic field, establishing a second magneticfield at right angles to the direction of the first magnetic field, bothfields being located in the same locality of exploration, and the secondmagnetic field having a frequency related to the strength of thefirst-mentioned magnetic field in accordance with the formula f=2.80[H:10] in which formula is the frequency of the first-mentioned field inmegacycles per second and H is the strength of the second-mentionedfield in gauss, and detecting magnetic fluctuations due to the presenceof said material in said magnetic fields.

10. A process as defined in claim 9, in which the second-mentioned fieldhas a frequency of from 0.1 to 10,000 megacycles per second.

11. A process as defined in claim 9, in which the second-mentioned fieldhas a frequency of from 0.25 to 250 megacycles per second and thefrequency-strength relationships of the two fields is in accordance withthe formula 12. A process of geophysical exploration for inaccessiblesubterranean deposits of a material from the group consisting ofpetroleum, coal, natural bitumen, partially carbonized animal andvegetable matter and carbonaceous geological deposits, which processcomprises estab lishing a first magnetic field in a subterraneanlocality to be explored, varying the strength of said magnetic field,establishing a second magnetic field at right angles to the direction ofthe first magnetic field, both fields being located in the samesubterranean locality of exploration, and the second magnetic fieldhaving a frequency related to the strength of the first-mentionedmagnetic field in accordance with the formula f=2.80[H :10] in whichformula 1 is the frequency of the first-mentioned field in megacyclesper second and H is the strength of the second-mentioned field in gauss,and detecting the presence of a third electromagnetic field produced bythe presence of said material in the locality being explored.

13. A process of geophysical exploration for inaccessible deposits of acarbonaceous material to detect said carbonaceous material, whichprocess comprises establishing a first subterranean magnetic field,varying the strength of said magnetic field until it is brought into theregion of resonance of electrons present in said carbonaceous materialat a selected frequency of operation, establishing a second magneticfield at right angles to and in the same subterranean locality as thesaid first magnetic field and having a frequency the same as saidselected frequency, and inducing in a circuit above the surface of theearth a current due to resonance existing in a carbonaceous material insaid subterranean locality and arising when the strength of said secondmagnetic field is altered as a result of such resonance, said currentbeing indicative of the presence of said carbonaceous material at saidsubterranean locality.

14. A process of geophysical exploration for inaccessible deposits of acarbonaceous material, which process comprises establishing a first beamof electromagnetic radiation in the region to be explored, having thestrength of its magnetic component within the region of resonance ofsaid carbonaceous material at a selected frequency of operation,establishing a second beam of electromagnetic radiation intersecting thefirst-mentioned beam at a selected depth of the region to be exploredand having a frequency within the region of resonance of thecarbonaceous material acted on by the eanths magnetic field, a portionof said second beam being reflected from at least one subterraneanposition and detecting variations in the reflected portion of saidsecond beam due to resonance caused by the presence of said carbonaceousmaterial in the locality of said radiation fields.

15. A process of geophysical exploration for inaccessi ble deposits of acarbonaceous material, which process comprises the steps of establishingin the location to be explored a first magnetic field varying at a firstselected frequency, establishing in said location a second magneticfield substantially perpendicular to that of the firstmentioned magneticfield and varying at a second selected frequency, and detecting thepresence of another magnetic field produced by said carbonaceousmaterial due to the presence of said carbonaceous material in thelocality where the said second-mentioned magnetic field is substantiallyperpendicular to the first-mentioned mag netic field, said othermagnetic field varying at a frequency chosen from the class consistingof the first frequency plus an integral multiple of the second frequencyand the first frequency minus an integral multiple of the secondfrequency.

16. Geophysical exploration apparatus for detecting inaccessiblesubterranean deposits of a carbonaceous material including: a firstfield-producing means for producing a magnetic field directed into theearth in a first direction; a second field-producing means for producinga magnetic field in the earth in a second direction substantially atright angles to said first direction, both of said fields being presentin a subterranean location suspected of containing a deposit of saidcarbonaceous material, and said second field-producing means including adirectional antenna for producing a radiation field with a magneticcomponent in said second direction; means for varying thestrength-frequency relationship of said fields through a zone includingthe region of electron resonance of such carbonaceous material at suchlocation; and detecting means electrically connected to said antenna fordetecting such resonance.

17. Apparatus as defined in claim 16, in which said secondfield-producing means includes a first directional antenna for producinga radiation field directed into the earth with a magnetic component insaid second di rection to produce a polarized beam subject to Faradayrotation, and including a second directional antenna responsive to therotated direction of polarization.

18. Geophysical exploration apparatus including: a first directionalantenna and means for supplying thereto a high frequency energizingpotential, said first antenna being beamed into the earth along a firstaxis to create a magnetic field consisting of a steady componentcontributed by the earths magnetic field and an alternating componentcontributed by said beam from said first antenna; a second directionalantenna and means for supplying thereto a high frequency energizingpotential, said second antenna being beamed into the earth along asecond axis in such direction that said beams intersecting in asubterranean location under test and the magnetic field created therebyis substantially perpendicular to the first-mentioned magnetic field, aportion of the beam from said second antenna being reflected fromsubterranean structures; and a detector responsive to such reflectedportion.

19. A process of detecting the presence of petroleum in apetroleum-containing mud, which process comprises subjecting said mud totwo magnetic fields at right angles to each other, one of said magneticfields having a frequency of from 0.1 to 10,000 megacycles per secondand the other of said magnetic fields having a strength related to thefrequency of the first-mentioned magnetic field in accordance with theformula in which formula f is the frequnecy of the first-mentioned fieldin megacycles per second and H is the strength of the second-mentionedfield in gauss, and detecting magnetic fluctuations due to the presenceof petroleum in said mud within said magnetic fields.

20. A process of detecting the presence of petroleum in apetroleum-containing mud, which process comprises subjecting said mud totwo magnetic fields at right angles to each other, both fields beinglocated in the same cality as the petroleum mud, one of said fieldsbeing a unidirectionally pulsating field and the other of said fieldsbeing an alternating field, varying the strength of saidunidirectionally pulsating magnetic field, the strength frequencyrelationship of said fields being in accordance with the formulaf=2.80[H :10] in which formula H is the strength of the said unidirectionally pulsating field in gauss and f is the frequency of thealternating field in megacycles per second, and de- 16 tecting magneticfluctuations due to the presence of petroleum in said mud within saidmagnetic fields.

21. A process of geophysical exploration for inaccessible subterraneandeposits of petroleum, which process comprises establishing a secondelectromagnetic field at right angles to the direction of a firstmagnetic field in an inaccessible subterranean region of exploration,the second field having a frequency related to the strength of the saidfirst field indicated by the formula f:2.80[H i10] in which formula, 1is the frequency of the said second field in megacycles per second and His the strength of the said first field in gauss, and detecting a changein the energy content of said second electromagnetic field due to thepresence of petroleum in said fields.

22. In the art of exploring the earths crust for detection ofcarbonaceous materials, a process using two magnetic fields crossingeach other at a subterranean position, said process being characterizedby the steps of varying the strength frequency relationship of the twofields in accordance with the formula:

in which formula is the frequency of one of said two fields inmegacycles per second and H is the strength of the other of said fieldsin gauss, to produce magnetic fluctuations as a result of the presenceof said carbonaceous material in the subterranean zone of explorationand detecting said fluctuations as an indication of the presence of saidcarbonaceous material in said zone.

23. The process as defined in claim 22, in which one of said magneticfields is the earths field.

24. A process as defined in claim 22, in which one of said magneticfields is an induction field and the other of said magnetic fields is aradiation field.

25. A process as defined in claim 22, in which both of said fields areinduction fields.

26. A process as defined in claim 22, in which both of said fields areradiation fields.

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:UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,060,371 October 23, 1962 Jonathan Townsend. et. a1,

Column 5, line 3, strike out "15" column 9, line 43, for "centering"read entering column 10, line 48, for "procession" read precessioncolumn 11, line 62, for "methodp reviously" read methgd previouslySigned and sealed this 26th day of March 1963.,

(SEAL) Attest:

ESTON G. JOHNSON DAVID L. LADD Attesting Officer Commissioner of Patents

